Here is a really simple method to control and maintain the exact water level in your aquarium.
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Step 1: Acquiring Resources
Drill and drill bits
(1) large airtight container (I used a 1 gal tea jug), preferably made of thicker plastic (not milk jugs). Free
(1) vertical or horizontal float valve. this will depend on your hood, mine has a tall vertical area above the water, so I used a horizontal switch. ~$8 on Amazon.
(~8ft) some lengths of airline tubing. you need enough to go from the air pump to the jug then from the jug to the valve. Free
Optional: inline airline valve. $3-4 on amazon.
Step 2: Preparing the Hood and Jug
You will need to drill two holes slightly smaller than the outer diameter of the air tubing in the lid of your jug. One of these is for the incoming air, the other is for the outgoing water.
Next, drill a hole big enough for the valve to go through your hood (mine is a 12 gallon Eclipse system).
Optional step: make a plastic water deflector out of the curved portion of a milk carton so the water runs away from the lip. (see next step).
Step 3: Connecting Everything and Test
Now, force the tube from the air pump into the cap, just a little bit, see picture. then pull the other tube through the second hole so that it will reach the bottom of the jug. If desired, you can seal the tubes into the cap, but I have been using this for the better part of a year, and have not needed to seal/glue them in.
Place the valve in the hood hole and add the deflector on the water side if desired. Now connect the tube coming from the jug and tighten, use an inline valve near the float valve if desired.
Add water and you're all set! the pressure from the air pump will pressurize the jug and if your water level drops in the aquarium, it opens the float valve. This allows the water to leave the jug and trickle into the tank.
Adjust the float arm to seal the valve at the desired water level, this takes a bit of tinkering but its not too bad.
Other notes: For this to work, you need either a separate air pump, or in my case, a secondary output that does not feed into a low resistance air stone or what have you.
This has served me extremely well, its dead simple and all I have to do is add water to the jug every time I do a water change.
Stay tuned for an electronic version coming soon on my new 60 gallon tank!